I have seen many people asking on forums how to buy games, game items or applications from Google Play store without using debit/credit card. Those who are asking the questions are underage or those who are more keen to online safety. In this case, Google play gift card solve your problem as they are easily available in local stores in many countries now.
You can check the available local stores online, and buy them with the exchange of currency. This is simple as you buy a bread butter for your daily need. After purchasing the Google Play gift card, you need to redeem it.
You can order a prepaid card and use it as a credit card; many firms are providing rebates on a card that works like a credit card. Such cards are activated for a particular amount and each time you apply it the amount is deducted, and that card preserves a balance until it is consumed.
Now You have a gift card bought, and You can use your Google Play Gift card towards the purchase of applications or game items (i.e. gems, credit, coins, etc).
To do this:
First, Be sure to redeem your gift card in your main Google Play Store account. If you need guidance on how to redeem, go through this video.
Secondly, confirm that you have the least amount to get your in-app purchase (see sample screenshot). If you face any problem with any in-app purchase, feel free to write your Queries with in-app purchase Help Center guide to troubleshoot the problem.
There are many apps in play store giving out free google play codes when you complete small tasks or surveys offered by the application or advertisers. This way is for them who do not want to spend a buck for such digital currency. To be honest, these tasks are very time consuming and sometimes does not worth your time. Make sure you choose the right app based on good reviews as some of the apps will show you your hard earn credits but then suspend your account for an unusual reason.